Carbon dioxide-sensing airway products and technique for using the same

ABSTRACT

An airway device is provided that may track the flow of respiratory gases through the device with sensing elements at a plurality of locations along the gas flow path of the device. Such a device may be useful for assessing a variety of clinical states, for adjusting patient ventilator settings, or for determining whether or not an airway device has been properly inserted into a patient airway.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to medical devices and, moreparticularly, to patient ventilation devices, such as breathing circuitsand tracheal tubes.

2. Description of the Related Art

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present invention,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

In the field of medicine, doctors often desire to monitor certainphysiological characteristics of their patients. Accordingly, a widevariety of devices have been developed for monitoring many suchcharacteristics of a patient. Such devices provide doctors and otherhealthcare personnel with the information they need to provide the bestpossible healthcare for their patients. As a result, such monitoringdevices have become an indispensable part of modern medicine.

Physiological characteristics that physicians may desire to monitorinclude constituents of the blood and tissue, such as oxygen and carbondioxide. For example, abnormal levels of carbon dioxide in the blood ortissue may be related to poor perfusion. Thus, assessment of carbondioxide levels may be useful for diagnosing a variety of clinical statesrelated to poor perfusion. Carbon dioxide and other blood constituentsmay be directly measured by taking a blood sample, or may be indirectlymeasured by assessing the concentration of those constituents in thetissue or respiratory gases. For example, carbon dioxide in thebloodstream equilibrates rapidly with carbon dioxide in the lungs, andthe partial pressure of the carbon dioxide in the lungs approaches theamount in the blood during each breath. Accordingly, physicians oftenmonitor respiratory gases during breathing in order to estimate thecarbon dioxide levels in the blood.

In the course of treating a patient, a tube or other medical device maybe used to control the flow of air, food, fluids, or other substancesinto the patient. For example, medical devices, such as tracheal tubes,may be used to control the flow of one or more substances into or out ofa patient. Such tracheal tubes may include endotracheal (ET) tubes ortracheostomy tubes. In this way, substances can only flow through thepassage via the tube or other medical device, allowing a medicalpractitioner to maintain control over the type and amount of substancesflowing into and out of the patient. Such airway devices may be part ofa breathing circuit that allows a physician to facilitate mechanicalventilation of the patient.

In certain instances, it may be advantageous to assess carbon dioxide inrespiratory gases that are flowing through airway devices. The carbondioxide levels in such gases are generally less contaminated byenvironmental gases because the airway devices provide at least apartial barrier to the egress or ingress of gas. Further, suchinformation is useful to a healthcare practitioner to determine whetherthe airway device is transferring sufficient respiratory gas to thelungs or to determine whether the patient is metabolizing therespiratory gas and producing the expected levels of carbon dioxide orother volatile metabolites. Thus, sampling carbon dioxide in an airwaydevice may provide a useful method of assessing physiological carbondioxide levels.

SUMMARY

Certain aspects commensurate in scope with the originally claimedinvention are set forth below. It should be understood that theseaspects are presented merely to provide the reader with a brief summaryof certain forms that the invention might take, and that these aspectsare not intended to limit the scope of the invention. Indeed, theinvention may encompass a variety of aspects that may not be set forthbelow.

There is provided a medical device that includes a conduit adapted totransfer a gas to or from a patient; and a plurality of sensingcomponents associated with a respective plurality of locations on theconduit, wherein the plurality of sensing components is adapted toprovide a signal related to a carbon dioxide gas in the conduit at therespective plurality of locations.

There is provided a system that includes a conduit adapted to transfer agas to or from a patient; a plurality of sensing components associatedwith a respective plurality of locations on the conduit, wherein theplurality of sensing components is adapted to provide a signal relatedto a carbon dioxide gas in the conduit at the respective plurality oflocations; and a monitor adapted to be operatively coupled to theplurality of sensing components.

There is provided a method of manufacturing a medical device thatincludes providing a conduit adapted to transfer a gas to or from apatient; and providing a plurality of sensing components associated witha respective plurality of locations on the conduit, wherein theplurality of sensing components is adapted to provide a signal relatedto a carbon dioxide gas in the conduit at the respective plurality oflocations.

There is provided a medical device that includes: a conduit adapted totransfer a gas to or from a patient; and a contiguous sensing componentdisposed along at least a portion of the conduit, wherein the contiguoussensing component is adapted to provide an indication of a carbondioxide gas in the conduit along the portion of the conduit.

There is provided a system that includes a conduit adapted to transfer agas to or from a patient; and a contiguous sensing component disposedalong at least a portion of the conduit, wherein the contiguous sensingcomponent is adapted to provide an indication of a carbon dioxide gas inthe conduit along the portion of the conduit; and a monitor adapted tobe operatively coupled to the contiguous sensing element at each of theplurality of locations.

There is provided a method of manufacturing a medical device thatincludes providing a conduit adapted to transfer a gas to or from apatient; and a contiguous sensing component disposed along at least aportion of the conduit, wherein the contiguous sensing component isadapted to provide an indication of a carbon dioxide gas in the conduitalong the portion of the conduit.

There is provided a medical system adapted to determine a change in anon-gas-exchanging physiologic volume in a ventilated patient thatincludes a processor adapted to: receive signals from a plurality ofcarbon dioxide sensors disposed on a patient breathing circuit;determine a concentration of carbon dioxide gas over time at a firstlocation in a patient breathing circuit; determine a concentration ofcarbon dioxide gas over time at a second location in the patientbreathing circuit; determine the transit time of the carbon dioxide gasbetween the two locations; determine the volume of the breathing circuitbetween the first and second locations based on the cross-sectional areaof the breathing circuit and the distance between the two locations; anddetermine the flow rate over time of the carbon dioxide gas between thefirst location and the second location in the patient breathing circuitbased on the determined volume and transit time, wherein a change innon-gas-exchanging physiologic volume is indicated by a change in thetotal volume exhaled while the carbon dioxide gas is increasing at thestart of exhalation.

There is also provided a computer readable medium with instructions fordetermining a change in a non-gas-exchanging physiologic volume in aventilated patient that includes: code for receiving signals from aplurality of carbon dioxide sensors disposed on a patient breathingcircuit; code for determining a concentration of carbon dioxide gas overtime at a first location in a patient breathing circuit; code fordetermining a concentration of carbon dioxide gas over time at a secondlocation in the patient breathing circuit; code for determining thetransit time of the carbon dioxide gas between the two locations; codefor determining the volume of the breathing circuit between the firstand second locations based on the cross-sectional area of the breathingcircuit and the distance between the two locations; and code fordetermining the flow rate over time of the carbon dioxide gas betweenthe first location and the second location in the patient breathingcircuit based on the determined volume and transit time, wherein achange in non-gas-exchanging physiologic volume is indicated by a changein the total volume exhaled while the carbon dioxide gas is increasingat the start of exhalation.

There is also provided a method of determining a change in anon-gas-exchanging physiologic volume in a ventilated patient thatincludes: determining a concentration of carbon dioxide gas over time ata first location in a patient breathing circuit; determining aconcentration of carbon dioxide gas over time at a second location inthe patient breathing circuit; determining the transit time of thecarbon dioxide gas between the two locations; determining the volume ofthe breathing circuit between the first and second locations based onthe cross-sectional area of the breathing circuit and the distancebetween the two locations; and determining the flow rate over time ofthe carbon dioxide gas between the first location and the secondlocation in the patient breathing circuit based on the determined volumeand transit time, wherein a change in non-gas-exchanging physiologicvolume is indicated by a change in the total volume exhaled while thecarbon dioxide gas is increasing at the start of exhalation.

There is provided a multi-lumen intubation tube that includes a conduitadapted to transfer gas to a patient's lungs that includes a first lumenand a second lumen; a first carbon dioxide sensing component disposed onthe first lumen; and a second carbon dioxide sensing component disposedon the second lumen.

There is also provided a method of manufacturing a multi-lumenintubation tube that includes: providing a conduit adapted to transfergas to a patient's lungs comprising a first lumen and a second lumen;providing a first carbon dioxide sensing component disposed on the firstlumen; and providing a second carbon dioxide sensing component disposedon the second lumen.

There is provided a method of determining which lumen is active in amulti-lumen tube that includes inserting a multi-lumen tube into apatient's airway; determining a concentration of carbon dioxide gas at alocation in a first lumen; and determining a concentration of carbondioxide gas at a location in a second lumen, wherein the lumen with thehigher concentration of carbon dioxide gas is the active lumen.

There is also provided a system that includes: a conduit adapted totransfer gas to or from a patient's lungs; an inflatable balloon cuffdisposed on the conduit; and at least one carbon dioxide sensingcomponent disposed on the inflatable balloon cuff; and a monitor adaptedto be operatively coupled to the plurality of sensing components.

There is also provided a method of manufacturing a medical device thatincludes: providing a conduit adapted to transfer a gas to or from apatient; providing an inflatable balloon cuff disposed on the conduit;and providing at least one carbon dioxide sensing component disposed onthe inflatable balloon cuff.

There is also provided a method that includes: receiving a signal fromat least one carbon dioxide sensing component disposed on an inflatableballoon cuff operatively connected to an endotracheal tube; andcorrelating the signal to a level of secretions on the cuff.

There is also provided a method that includes: receiving a first signalfrom a first carbon dioxide sensing component disposed on anendotracheal tube; receiving a second signal from a second carbondioxide sensing component disposed on an endotracheal tube; andcorrelating a difference between the first signal and the second signalto a level of secretions on a surface of the endotracheal tube.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention may become apparent upon reading thefollowing detailed description and upon reference to the drawings inwhich:

FIG. 1 illustrates an exemplary patient breathing circuit with multiplecarbon dioxide sensing components in accordance with aspects of thepresent technique;

FIG. 2 illustrates an exemplary endotracheal tube with multipleborosilicate carbon dioxide sensing components along the lumen of thetube.

FIG. 3 illustrates an exemplary endotracheal tube with a contiguousborosilicate-sensing component along the lumen of the tube;

FIG. 4 illustrates a cross-sectional view of an exemplary medicalconduit with a borosilicate carbon dioxide sensing layer;

FIG. 5 illustrates a cross-sectional view of an exemplary medicalconduit with borosilicate carbon dioxide sensing portions;

FIG. 6 illustrates an exemplary endotracheal tube with multipleside-sample lumens according to the present techniques;

FIG. 7 illustrates an exemplary endotracheal tube with a contiguouscarbon dioxide sensing component on the top of the inflatable cuff;

FIG. 8 illustrates Combitube with a carbon dioxide sensing component inthe tracheal lumen and the esophageal lumen; and

FIG. 9 is a flowchart for determining the physiological dead volumeusing an endotracheal tube or breathing circuit according to the presenttechniques.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

It is desirable to provide an airway device, such as an endotrachealtube or other medical device, which may sense carbon dioxideconcentration at multiple points along the device. Having a plurality ofcarbon dioxide sensing components allows carbon dioxide concentrationsto be tracked throughout a single breathing cycle as respiratory gasestravel through the device and encounter each sensing component. This mayallow more rapid monitoring of changes to carbon dioxide concentrationsthan typical devices that rely upon a single carbon dioxide sensor andtrack the carbon dioxide at a single point in each breathing cycle.Rapid monitoring of carbon dioxide concentration changes may providecertain advantages, such as more rapid detection of clinical states thatmay be related to the underlying carbon dioxide concentration changes.Further, providing carbon dioxide sensing components along the path of apatient airway device may allow a rapid visual signal that carbondioxide in expired gases is flowing through the airway as expected.Alternatively, the lack of such a visual signal may direct immediateattention to leaky airway connections, non-optimal ventilator settings,or a change in the physiological state of the patient.

In certain embodiments, the present techniques may be used inconjunction with any appropriate medical device, including a feedingtube, an endotracheal tube, a tracheostomy tube, a circuit, an airwayaccessory, a connector, an adapter, a filter, a humidifier, a nebulizer,nasal cannula, or a laryngeal mask. The present techniques may also beused to monitor any patient benefiting from mechanical ventilation.Further, the devices and techniques provided herein may be used tomonitor a human patient, such as a trauma victim, an intubated patient,a patient with a tracheostomy, an anesthetized patient, a cardiac arrestvictim, a patient suffering from airway obstruction, or a patientsuffering from respiratory failure.

FIG. 1 illustrates a schematic of an exemplary patient breathing circuit12 with multiple carbon dioxide sensing components 10 a, 10 b, and 10 c,discussed in more detail herein. The sensing components, genericallyreferred to by the reference numeral 10, may be used in conjunction witha carbon dioxide monitoring system 11. The sensing components 10 may belocated along any point in the breathing circuit 12. For example, thesensing components may be located in the portion of the breathingcircuit 12 inserted into the patient, or in any of the associated tubingor connectors. For example, in one embodiment, a sensing component 10may be located directly after the carbon dioxide filter 24 in theairflow circuit in order to assess the quality of the carbon dioxidefilter 24. In other embodiments, the sensing components 10 may beadapted to determine the level of carbon dioxide in an anesthetic gasmixture being delivered to the patient. It should be appreciated thatthe sensing components 10 may be coupled to the monitor 11 with cables13 or may be coupled to a transmission device (not shown) to facilitatewireless transmission between the sensing components 10 and the monitor11. The monitor 11 may be any suitable carbon dioxide monitor, such asthose available from Nellcor Puritan Bennett Inc. Furthermore, toupgrade conventional carbon dioxide monitoring provided by the monitor11 to provide additional functions, the monitor 11 may be coupled to amulti-parameter patient monitor (not shown).

The breathing circuit 12 may also include a Y-shaped respiratory circuit16 that allows one-way flow of expired gases away from the patient andone-way flow of inspired gases from a source gas supply 18 towards thepatient. The one-way flow of gases through the Y-shaped respiratorycircuit 16 may be achieved through the use of in-line one-way valves 20aand 20b. The source gas may include respiratory gas mixtures andanesthetic/therapeutic components such as anesthetic agents, nitricoxide, radioactively tagged particles and/or a variety of other gaseousagents. It will be appreciated that the expired gas stream may alsoinclude a combination of respiratory gases and anesthetic/therapeuticgases. The Y-shaped respiratory circuit 16 may also include a vent 21and/or a gas reservoir 22 to relieve excess volume or pressure in thebreathing circuit 12. The Y-shaped respiratory circuit 16 may alsoinclude a carbon dioxide filter 24 to remove carbon dioxide frombreathing circuit 12 before the fresh source gas is added to theairflow.

The Y-shaped respiratory circuit 16 may be connected to the patient withany suitable airway device, such as a tracheal tube 14, as shown. TheY-shaped respiratory circuit 16 may include standard medical tubing madefrom suitable materials such as polyurethane, polyvinyl chloride (PVC),polyethylene teraphthalate (PETP), lowdensity polyethylene (LDPE),polypropylene, silicone, neoprene, or polyisoprene.

The breathing circuit 12 may be incorporated into systems thatfacilitate positive pressure ventilation of a patient, such as aventilator 26. Such systems may typically include a monitor and/or acontroller. The controller may be a digital controller, a computer, anelectromechanical programmable controller, or any other control system.In certain embodiments (not shown), the breathing circuit 12 may includeone or more temperature sensors that provide information to the monitor11 about the temperature of gas flowing through the breathing circuit12. Suitable temperature sensors according to the present techniquesinclude any suitable medical grade temperature sensor, such asresistance-based temperature sensors and infrared temperature sensorsavailable from Thermometrics (Plainville, Conn.). In other embodiments,the breathing circuit may include humidity and ambient pressure sensors.For example, in certain embodiments, it may be advantageous to determinespectral changes as a function of pressure. In such embodiments, abreathing circuit 12 may include barometric pressure sensors todetermine ambient pressure or intra-circuit pressure.) In otherembodiments, it may be advantageous for a healthcare worker to manuallyinput ambient conditions, such as the temperature of room or the patienttemperature, into the monitoring system.

As depicted, the endotracheal tube 28 may also include an inflatablecuff 34 that may be inflated to form a seal against the trachea walls.Typically, the cuff 34 is disposed, adhesively or otherwise, towards thedistal end 36 of the conduit 32. The cuff 34 may be inflated anddeflated via an inflation lumen 38 in communication with the cuff 34,typically through a hole or a notch in the conduit 32. The cuff 34 has aproximal opening 40 and a distal opening 42 formed in the cuff walls toaccommodate the conduit 32. The cuff 34 may be formed from materialshaving suitable mechanical properties (such as puncture resistance, pinhole resistance, tensile strength), chemical properties (such as forminga suitable bond to the tube 32), and biocompatibility. In oneembodiment, the walls of the inflatable cuff 34 are made of polyurethanehaving suitable mechanical and chemical properties. An example of asuitable polyurethane is Dow Pellethane® 2363-90A. In anotherembodiment, the walls of the inflatable cuff 34 are made of a suitablepolyvinyl chloride (PVC). Suitable materials may also includepolyethylene teraphthalate (PETP), low-density polyethylene (LDPE),polypropylene, silicone, neoprene, or polyisoprene.

In addition to carbon dioxide monitoring, sensing components 10, asprovided herein, may be used to monitor oxygen, carbon monoxide,volatile organic compounds such as ethanol, metabolic trace gases suchas acetone, or anesthetic gases such as isoflurane, halothane,desflurane, sevoflurane, and enflurane. For example, respiratory gasesassociated with an acute or chronic disease state may be monitored usingthe present techniques.

In other embodiments (not shown), the sensing components 10 may beincorporated into systems that sense carbon dioxide transcutaneously.For example, in such a system, a gas chamber type collection system maybe placed on a patient's skin to capture any gas that may diffuse awayfrom the skin to a distal site. The carbon dioxide levels in the gas maybe used as a surrogate marker for carbon dioxide levels in the blood, asblood carbon dioxide may diffuse through the tissue. The skin may beheated in order to facilitate the diffusion of gas through the tissue.The sensing components 10 may be located at multiple locations along thesystem and a change in the levels of carbon dioxide along the system mayindicate a change in clinical state, such as poisoning.

Sensing components 10 as described herein may include any appropriatesensor or sensor element for assessing expired carbon dioxide, includingchemical, electrical, optical, non-optical, quantum-restricted,electrochemical, enzymatic, spectrophotometric, fluorescent, orchemiluminescent indicators or transducers. In certain embodiments, thesensing component 10 may include optical components, e.g., an emitterand detector pair that may be of any suitable type. For example, theemitter may be one or more light emitting diodes adapted to transmit oneor more wavelengths of light in the red to infrared range, and thedetector may be one or more photodetectors selected to receive light inthe range or ranges emitted from the emitter. Alternatively, an emittermay also be a laser diode or a vertical cavity surface emitting laser(VCSEL). An emitter and detector may also include optical fiber sensingcomponents. An emitter may include a broadband or “white light” source,in which case the detector could include any of a variety of elementsfor selecting specific wavelengths, for example, reflective orrefractive elements or interferometers. These kinds of emitters and/ordetectors would typically be coupled to the rigid or rigidified sensorvia fiber optics. Alternatively, a sensing component 10 may sense lightdetected through the respiratory gas at a different wavelength from thelight emitted into the respiratory gas. Such sensors may be adapted tosense fluorescence, phosphorescence, Raman scattering, Rayleighscattering and multi-photon events or photoacoustic effects. It shouldbe understood that, as used herein, the term “light” may refer to one ormore of ultrasound, radio, microwave, millimeter wave, infrared,visible, ultraviolet, gamma ray or X-ray electromagnetic radiation, andmay also include any wavelength within the ultrasound, radio, microwave,millimeter wave, infrared, visible, ultraviolet, gamma ray or X-rayspectra.

The sensing component 10 may be an electrochemical transducer, which maybe adapted to detect and measure changes in ambient chemical parametersinduced by the presence of critical amounts of carbon dioxide. In oneembodiment, the sensing component 10 may include a sensor that employscyclic voltammetry for carbon dioxide detection. Such sensors areavailable from Giner, Inc., Newton, Mass. For example, the sensingcomponent 10 may be a thick film catalyst sensor utilizing a protonexchange membrane. Such a sensing component 10 may include thick filmscreen printed electrodes and an electrochemically reversible metaloxide catalysts. Appropriate catalysts include MO, M₂O₃, MO₂, where M isa metal that is any suitable metal, including platinum, ruthenium, oriridium. Generally, such sensors operate by sensing chemical reactionscaused by proton dissociation from water in which carbon dioxide isdissolved. Dissociated water protons may electrochemically reduce ametal oxide layer of the sensor. The electrochemical reduction of themetal oxide will result in generation of an electrical current, whichvaries in response to the degree of electrochemical reduction.

In another embodiment, the sensing component 10 may includequantum-restricted components, including carbon nanotubes, buckeyballs,or quantum dots. Generally, quantum-restricted components may be coatedor otherwise modified with a compound that is sensitive to therespiratory gas of interest. Interaction of the respiratory gas with thecompound may affect the electrical, optical, thermal, or physicalproperties of the quantum-restricted components such that a signal mayresult. In one such example, carbon nanotubes may be coated with acarbon dioxide-sensitive compound or polymer, such as apolyethyleneimine and starch polymer. Carbon dioxide may combine withprimary and tertiary amines in the polyethyleneimine and starch polymercoating to form carbamates. The chemical reaction alters the chargetransfer to the carbon nanotubes and results in an electrical signal.Other suitable polymer coatings may be adapted to sense otherrespiratory gases of interest, such as oxygen or carbon monoxide. Inother embodiments, the quantum-restricted component may include abinding molecule, such as a receptor or an enzyme that is specific forthe respiratory gas of interest. One such molecule may include carbonicanhydrase. Binding of the respiratory gas to its receptor may affect adownstream response that may result in a change in the electricalproperties of a quantum-restricted component.

The sensing component 10 may also include a semi-conductive sensingelement, such as a field-effect transistor (FET) or an ion-sensitivefield-effect transistor (ISFET). An ISFET may include a silicon dioxidegate for a pH selective membrane. Such a sensor may be adapted to sensedownstream changes in hydrogen ion concentration in response to changesin carbon dioxide or other respiratory gas concentrations. In certainembodiments, the semi-conductive sensing element may be a film.

Alternatively, the sensing component 10 may include an active ingredientof the indicating element, for example the active ingredient involved inproviding the required response signal when exposed to a givenconcentration of carbon dioxide or other constituents. The activeingredient may be any indicator that is sensitive to the presence ofcarbon dioxide and that is capable of being calibrated to give aresponse signal corresponding to a given predetermined concentration ofcarbon dioxide. The signal may be visual, e.g., a change in color, orelectrical. Indicators that provide a color change in a presence ofcarbon dioxide may include chromogenic pH-sensitive indicators andoxidation/reduction indicators.

A chromogenic pH-sensitive indicator may provide a color change uponexposure to a given concentration of carbon dioxide or other metabolitesin the presence of other ingredients of the element that provide theappropriate chemical conditions to induce the required color change. Forsuch an indicator to be capable of giving a determination of carbondioxide, it is typically used in combination with a suitable base thatprovides an alkaline solution. The hydroxyl ions or amine residuespresent in the alkaline solution react chemically with carbon dioxide toproduce a carbonate, bicarbonate and/or carbamate moiety. The resultingreaction depletes the hydroxyl ion or amine at the interface and thuslowers the pH at the surface of the component impregnated with theindicating element. The lowering of the pH causes a color change in theindicator.

Chromogenic pH-sensitive indicators, according to the presenttechniques, may include metacresol purple, thymol blue, cresol red,phenol red, xylenol blue, a 3:1 mixture of cresol red and thymol blue,bromthymol blue, neutral red, phenolphthalein, rosolic acid,alpha-naphtholphthalein and orange I. Examples of other indicators thatmay be used include bromcresol purple, bromphenol red, p-nitrophenol,m-nitrophenol, curcumin, quinoline blue, thymolphthalein and mixturesthereof. Suitable bases include sodium carbonate, lithium hydroxide,sodium hydroxide, potassium hydroxide, potassium carbonate, sodiumbarbitol, tribasic sodium phosphate, dibasic sodium phosphate, potassiumacetate, monoethanolamine, diethanolamine and piperidine.

The sensing component 10 may also include an enzyme-based detectionsystem. For example, one such enzyme may be carbonic anhydrase, which isan enzyme that assists interconversion of carbon dioxide and water intocarbonic acid, protons, and bicarbonate ions. As described above, thisreaction lowers the pH at the surface of the component impregnated withthe indicating element. The lowering of the pH may cause a color changein the indicator. Another such enzyme-based detection system is anenzyme linked immunosorbent assay (ELISA). For example, such an assaymay be appropriate when assessing tissue proteins. Thus, the indicatorelement may include a primary antibody specific for the tissue proteinof interest, and a labeled secondary binding ligand or antibody, or asecondary binding ligand or antibody in conjunction with a labeledtertiary antibody or third binding ligand. The label may be an enzymethat will generate color development upon incubating with an appropriatechromogenic substrate. Suitable enzymes include urease, glucose oxidase,alkaline phosphatase or hydrogen peroxidase.

A chemical indicator may be used in conjunction with an electrical orelectronic device that is adapted to detect and measure changes in theambient chemical parameters induced by the presence of critical amountsof carbon dioxide. For example, optical fiber carbon dioxide sensingcomponents 10 may be used to convert a change in a chemical indicator toa quantitative measurement of carbon dioxide in the sample. Generally,such sensing components 10 operate by directing light of a predeterminedwavelength from an external source through the optical fiber to impingethe chemical indicator. The intensity of the emitted fluorescent lightreturning along the fiber is directly related to the concentration ofcarbon dioxide in the sample, as a result of the pH-sensitive indicatormaterial present at the fiber tip (i.e., the pH of the indicatorsolution is directly related to carbon dioxide concentration, as aresult of carbonic acid formation). The emitted light is carried by theoptical fiber to a device where it is detected and convertedelectronically to a carbon dioxide concentration value. The sensingcomponent 10 may additionally have a reference dye present in theindicator composition. The intensity of the light emitted from thereference dye may be used to compensate, via ratioing, the signalobtained from the indicator. Other components may be incorporated intothe indicator composition including surfactants, antioxidants andultraviolet stabilizers. The sensing component 10 may be formed from anyappropriate substrate. For example, the sensing component 10 may befilter paper, which may be soaked in, dipped in, or otherwise exposed tothe appropriate carbon dioxide-sensing compounds. In certainembodiments, the filter paper may be dipped into a solution containingthe indicating compounds on only one side. The sensing component 10 mayalso be polysulfone, polyproplylene, or other polymer substrates. Thesensing component may be a thin film, or a thicker substrate. A thickersubstrate may lead to a slower response time, which may be advantageousin situations in which a sensor is monitoring carbon dioxide levels overa longer period of time. Additionally, the sensing component may havepores of a variety of sizes.

In another embodiment, the sensing component 10 may include anartificial nose assembly. In such an embodiment, the respiratory gas maycontact an array of electrodes coated with polymers that havecharacteristic electrical properties. For example, the polymers maychange electrical resistance when contacted with specific materials.

In certain embodiments, the sensor sensing components 10 may includematerials that function as a selective barrier that are hydrophobic orotherwise water-resistant, but are permeable to carbon dioxide or otherrespiratory gases. Such a barrier may be of some advantage in the humidenvironment of the breathing circuit 12. In one embodiment, it isenvisioned that the ratio of water permeability to carbon dioxidepermeability of a selective barrier may be less than 10, and, in certainembodiments, the ratio may be less than 1. Suitable materials for aselective barrier include polymers, such as polytetrafluorethylene(PTFE). Other suitable materials include microporous polymer films, suchas those available from the Landec Corporation (Menlo Park, Calif.).Such microporous polymer films are formed from a polymer film base witha customizable crystalline polymeric coating that may be customized tobe highly permeable to carbon dioxide and relatively impermeable towater. The thickness of a selective barrier may be modified in order toachieve the desired rate of carbon dioxide perfusion and transducerresponse time. Generally, response times may be in the range ofvirtually instantaneous to less than 5 minutes. In certain embodiments,the response time is in the range of 5 seconds to 5 minutes. Where avery rapid response is desired, a thin film of the selective barrier,for example less than 0.2 mm in thickness, may be used. In certainembodiments, when a slower response is desired, a selective barrier mayrange from 0.2 mm to several millimeters in thickness. Additionally, theselective barrier may be formed with small pores that increase thecarbon dioxide permeability. The pores may be of a size of 0.01 toapproximately 10 microns, depending on the desired response time. In oneembodiment, the selective barrier may be a relatively thin PTFE materialsuch as plumber's tape (0.04 mm). In other embodiments, the selectivebarrier may be a PTFE material such as Gore-Tex® (W. L. Gore &Associates, Inc., Newark, Den.). Alternatively, the selective barrier 22may be formed from a combination of appropriate materials, such asmaterials that are heat-sealed or laminated to one another. For example,the selective barrier may include a PTFE layer with a pore size of 3microns and a second PTFE layer with a pore size of 0.1 microns.

A sensing component 10 may also include a borosilicate sensing elementsuch as those discussed in the U.S. patent application titled “CARBONDIOXIDE DETECTOR HAVING A BOROSILICATE SUBSTRATE” to Rafael Ostrowskiand Martin Debreczeny filed on Sep. 12, 2006, which is herebyincorporated by reference in its entirety herein. An example of amedical device appropriate for use with a borosilicate sensing elementis the endotracheal tube 28, depicted in FIG. 2. In this particularembodiment, the endotracheal tube 28 includes multiple borosilicatesensing components 30 disposed along the inside lumen of a conduit 32adapted to transfer air from the Y-shaped circuit 16, for example into apatient's lungs. The borosilicate sensing components 30 may include asubstrate 44 and an indicator solution, discussed in more detail herein.

The borosilicate sensing components 30 are generally adapted to changecolor in a reversible manner upon exposure to a critical threshold ofcarbon dioxide. Thus, during a patient respiratory cycle, respiratorygases from the lung enter the endotracheal tube 28 at the distal end andencounter the borosilicate sensing components 30 along the length of thetube. As gas containing expired carbon dioxide flows through theendotracheal tube, each of the borosilicate sensing components 30 inturn will respond to the increased local concentration of carbondioxide. If the concentration is greater than the critical threshold, acolor change will result. As the expired gas flows past each individualborosilicate sensing component 30, after a certain amount of time thecolor change may reverse as the local carbon dioxide concentration dropsaround an individual borosilicate-sensing component 30. Thus, the colorchange may occur as a wave along the endotracheal tube 28. As such, ahealthcare practitioner may track expired carbon dioxide in arespiratory gas mixture as it flows along the endotracheal tube 28 orany other airway device during an individual breath.

The borosilicate-sensing components 30 may be any suitable size orshape. In certain embodiments, it may be advantageous for theborosilicate sensing components 30 to be at least large enough to beseen with the naked eye. In other embodiments, the color change may bemonitored electronically. The borosilicate sensing components 30 may bespaced at any suitable distance along the endotracheal tube 28. Incertain embodiments, the borosilicate-sensing components 30 may bespaced at least 5 mm apart, at least 1 cm apart, or at least 10 cmapart. As the borosilicate sensing-components 30 in the intubatedportion of the endotracheal tube may not be visible to a healthcareworker, it may be advantageous in certain embodiments to couple theborosilicate-sensing components 30 to an optical sensor that is able todetect the indicator solution color change and provide a relatedelectrical signal. The optical sensor may be coupled to the monitor 11such that the electrical signal may be further processed. In certainembodiments (not shown) the borosilicate-sensing components 30 may bedisposed on the breathing circuit 12. In such an embodiment, a visualcolor change may provide information to a healthcare worker about theflow of carbon dioxide through the breathing circuit 12. However, itshould be understood that the borosilicate sensing components 30 in suchan embodiment may also include optical sensors that provide anelectrical signal to the monitor 11.

The substrate 44 may include any borosilicate-containing material.Specifically, it may include borosilicate fibers. These fibers may beproduced using any conventional methods, such as melt blowing andspinning. The substrate may include a mesh of borosilicate fibers. Morespecifically, it may include a thin, highly porous mesh to facilitaterapid infiltration of carbon dioxide gas into the substrate.

The borosilicate-containing substrate 44 may be sufficiently hydrophilicto allow the indicator solution to spread evenly over the substrate 44and be well absorbed when it is first applied. The indicator solutionmay then be dried, but still retain sufficient water to allow reactionwith carbon dioxide. However, the borosilicate substrate may also not beso hydrophobic that its shelf-life is compromised. Theborosilicate-containing material may also include an acrylic binder. Inspecific embodiments, this binder may be no more than 5% by weight orvolume of the total substrate without indicator. Metrigard® membranescontaining acrylic binder sold by Pall Corporation (New York) or asimilar acrylic binder may be used.

The indicator solution may contain an indicator, such as a chromogenicdye, in a solution. The indicator solution may be coated onto orimpregnated into the substrate 44. It may have a surface exposed to ornear air or gas within the sensing element 30. The indicator solutionmay be able to respond rapidly and positively to the presence or absenceof certain concentrations of carbon dioxide. More specifically, it maybe able to respond to concentrations of carbon dioxide normally presentin air respired from a human, such as between approximately 2% and 5% orhigher. The indicator solution may also be able to respond toconcentrations of carbon dioxide in air respired from a human withperfusion failure, such as concentrations between approximately 0.5% and2%. Finally, the indicator solution may show no response to carbondioxide concentrations normally present in external air or esophagealair, such as concentrations below approximately 0.5% and morespecifically, concentrations between 0.03% and 0.5%.

Response times to changing carbon dioxide levels in detected air may bebetween virtually instantaneous to about 20 seconds. Further, aborosilicate substrate 12 may exhibit virtually instantaneous responsetimes of less than 1 second, which is an improvement over typicalcolorimetric carbon dioxide detection systems. Response may include acalorimetric indication, such as change of the indicator from one colorto a very distinct second color. However, once the color begins tochange, the change from one color to the other color may be virtuallyinstantaneous as seen by the human eye. In order to attain the aboveresponse properties, the indicator in the indicator solution may have apK lower by 1.0-1.5 pH units than the pH of the indicator solution. Thisdifference allows the indicator solution not to change color instantlywhen exposed to air, allowing the detector system to be removed frompackaging then connected to another device, such as a resuscitator.However, due to a greater resistance to negative effects of air exposurewhen a borosilicate or borosilicate+acrylic substrate is used as opposedto cellulose filter paper, an indicator pK outside of this range maystill be acceptable. In general, any pK sufficient to allow the carbondioxide detector to remain exposed to room or outside air for at least15 minutes, at least 30 minutes, at least 60 minutes, or at least 120minutes without significant color change may be sufficient.

The indicator solution may include an alkaline solution containinghydroxyl ions or amine residues that react chemically with carbondioxide to form a carbonate and/or a bicarbonate or carbamate moiety.This reaction may be represented by the following equations:

CARBON DIOXIDE+H₂O

HCO₃ ⁻+H⁺  I

CARBON DIOXIDE+H₂O

CO₃ ²⁻+2H⁺  II

CARBON DIOXIDE+R₂NH

R₂NCOO⁻+H⁺

This reaction depletes the hydroxyl ion or amine at the interfacebetween the indicator solution and air and this lowers the pH at thesurface of the indicator solution where it is adjacent or nearlyadjacent to air. This depletion results in the diffusion of new basefrom elsewhere in the indicator solution to its surface to maintain asurface pH similar to that of the indicator solution overall.

More specifically, the concentration of OH⁻ or amine in the bulk of theindicator solution impregnated in or coated on the substrate 44 helpsdetermine the rate of diffusion of base to the surface of the indicatorsolution. The rate of the chemical reaction at this surface isdetermined by the nature of each specific reacting species. The rate ofreaction at the surface of the indicator solution may be expressed bythe equation R=K_(A)[CARBON DIOXIDE][A], where [x] represents theconcentration of a species in moles/liter and K_(A) is a constantspecific for reactant species A. In a specific embodiment, A is theindicator. The balance of base between the surface and remainder of theindicator solution is also influenced by the contact time between thesurface and the gas to which it is exposed, the composition of thesubstrate 44, which determines the diffusivity constant for A and thusthe rate of diffusion of A to the surface, and the concentration ofcarbon dioxide in the gas, which determines the rate of diffusion ofcarbon dioxide into or near the surface of the indicator where it mayreact with the indicator.

The concentration of OH⁻ or amine in the indicator solution, the rate ofthe chemical reaction, the contact time between the indicator surfaceand the gas, and the diffusivity constant for A may all be predeterminedby the manner in which the carbon dioxide detector is constructed andthe manner in which it is used. This leaves the concentration of carbondioxide in the gas the only variable parameter with significant effect,allowing for its measurement.

The concentration of OH⁻ or amine in the indicator solution and the rateof the chemical reaction may be selected such that the pH near thesurface of the indicator solution decreases sufficiently in the presenceof a certain concentration of carbon dioxide to cause a color change inthe indicator solution. For example, the color change may occur if theconcentration of carbon dioxide in the tested air is greater thanapproximately 2%. This color change may occur within 1 to 20 seconds ofexposure of carbon dioxide detector 10 to the air. In a specificexample, a concentration of OH⁻ sufficient to produce a pH of 9.6±0.2 inthe indicator solution is sufficient to provide this sensitivity.

As noted above, the indicator may have a pK sufficiently lower than thepH of the indicator solution so that a color change does not occur uponexposure to room or outside air for a certain time period. Exposure toair causes the pH at the surface of the indicator solution 14 togradually decrease, but if such decrease is sufficiently slow, thedesired time period without color change limitation may still be met.

The indicator used may affect which base is used to provide an alkalinethe indicator solution. For example, if the pK of the indicator is toolow, it is possible that with certain bases the pH of the indicator willnot drop low enough to cause a color change in the presence of anelevated carbon dioxide concentration. For example, when a sodiumhydroxide base is used, the carbonate reaction product is water solubleand also a base. This buffers a pH decrease and may prevent the pH fromreaching a level able to trigger a color change in the indicatingelement if the indicator has a low pK. Calcium hydroxide may be used asa base in embodiments of this description. Calcium hydroxide serves as asource of hydroxyl ions, but its carbonate reaction product with carbondioxide is insoluble and, therefore, unable to buffer the indicatorsolution against a decrease in pH. Thus calcium hydroxide may be usedwith indicators having relatively low pKs, such as metacresol purplerather than, for example, thymol blue or phenol phthalein. This alsoallows for increased resistance to color change when exposed to room orexternal air. However, the use of a borosilicate or borosilicate+acrylicin the substrate 44 may allow use of a buffering source of hydroxyl ionsin the indicator solution.

Various colorless compounds may be used to provide an alkaline theindicator solution. These include, but are not limited to calciumhydroxide, sodium carbonate, lithium hydroxide, sodium hydroxide,potassium hydroxide, magnesium hydroxide, potassium carbonate, sodiumbarbitol, tribasic sodium phosphate, dibasic sodium phosphate, potassiumacetate, monoethanolamine, diethanolamine, and piperidine. However, ifan acrylic-bound borosilicate is used as a substrate, no base may beneeded.

Various pH sensitive indicators may also be used in the indicatorsolution. These include, but are not limited to metacresol purple,thymol blue, cresol red, phenol red, xylenol blue, a 3:1 mixture ofcresol red and thymol blue, bromothymol blue, neutral red,phenolphthalein, rosolic acid, α-naphthelphthalein, and orange I. OtherpH indicators, the color change that occurs, and the relevant pH as wellas other information may be found in the CRC Handbook of Chemistry andPhysics, 8-17, 75th Edition 1994.

The indicator solution may also contain a hygroscopic, higlboiling,transparent, colorless, water-miscible liquid. This liquid may entrapsufficient water in the indicator solution when it is coated onto orimpregnated into the substrate 44 to allow reaction of the surface ofindicator 14 with carbon dioxide present in carbon dioxide detector 10.

Example hygroscopic, high-boiling, transparent, colorless,water-miscible liquids that may be used in the indicator solutioninclude, but are not limited to glycerol, propylene glycol, monoethyleneglycol, diethylene glycol, polyethylene glycol, and aliphatic alcohols.In specific embodiments, glycerol and propylene glycol or mixturesthereof may be used because of their antiseptic and non-toxicproperties. Acrylic binder used in some embodiments of the disclosurealso increases the hydrophobicity of the substrate 44 and may thusdecrease the need for a hygroscopic, highboiling, transparent,colorless, water-miscible liquid in the indicator solution.

The indicator solution may be in an aqueous solution, or it may not bein solution in water. It may require or benefit from the presence ofwater, or may function independently of water. The indicator solutionmay also be any type of chromogenic agent. For example, it may be achromogenic agent that does not go into solution in water, but thatnevertheless relies on nearby water.

When used, an acrylic binder provides a more basic environment for anindicator and also increases the hydrophobicity of the substrate. Abasic environment may help keep the color of the indicator appropriatein low carbon dioxide situations, such as less than 0.5%. Acrylic is anelectron rich compound, which makes it a good Bronstead and Lewis base.The resulting ability to accept protons from proton rich compounds andto donate a pair of electrons to electron poor compounds allows theindicator to remain unreacted. Enough carbonic acid is formed to affectthe indicator, but some of the acid is reacted by the acrylic.

A desired ratio of proton acceptance to compound concentration may bedetermined for different detectors. Varying the concentration of theacrylic binder will have an effect on the amount of carbonic acidavailable to react with the indicator when carbon dioxide is present inlarger amounts. Thus, carbon dioxide detectors 10 that also containacrylic binder in the substrate 44 may not use sodium carbonate becausethe binder itself may provide a more basic environment for theindicator. When acrylic binder is used, the final color of driedindicator may also be less sensitive to changes in the pH of theindicator solution. This may allow for a decrease in the amount ofindicator in the indicator solution by as much as approximately 66% ascompared to cellulose-based carbon dioxide detectors.

In a specific embodiment, the indicator solution may include 0.0169 g ofcresol red, 275 mL triethylene glycol, and 725 mL deionized water. Thisindicator solution may lack carbonate. The indicator solution may beimmobilized on the substrate 44 by drying, which removes a substantialamount of water. However, the reaction between the indicator and carbondioxide may utilize water. Therefore, some water may be absorbed by theindicator solution and/or the substrate 44 before use. For example,water may be absorbed from ambient air. In a specific embodiment,sufficient water may be absorbed in the time period required to removecarbon dioxide detector 10 from protective packaging and begin itsactual use. For example, sufficient water may be absorbed by theindicator solution in less than 10, 5 or 1 seconds after the opening ofany protective packaging.

The indicator solution may also be placed on the substrate 44 in variousother forms or using other methods. For example, it may be provided in ahydrogel. The substrate 44 may also be treated, for example by plasmatreatment, prior to administration of the indicator solution.

Use of a borosilicate substrate may result in desirable response timeand shelf life of a carbon dioxide detector, while retaining thecapacity of the detector to cycle from one color to another quickly frombreath to breath. For example, in some carbon dioxide detectors,reaction of the substrate with cresol red, which is used as a colorindicator, eventually changes the color indicator irreversibly frompurple to yellow. This change makes the detector color insensitive tothe presence or absence of carbon dioxide. As a result, the detectorsystem is no longer functional. Although packaging can help prevent thissensor aging, it nevertheless may limit shelf life. Borosilicatesubstrates do not react with cresol red. As a result, the same shelflife as is obtained with other substrates may be achieved withborosilicate and more cost effective packaging, or, a longer shelf lifeeven in the same packaging may be achieved. In certain embodiments, theshelf life of a borosilicate-based carbon dioxide detector may begreater than 5 years, great than 10 years, or greater than 14 years.Further, while the shelf life of a borosilicate-based carbon dioxidedetector may be greatly improved, the packaging employed may be reduced,due to the stability of the borosilicate-based carbon dioxide detector.While other calorimetric carbon dioxide detection systems may employdessicants to extend their shelf lives, a borosilicate-based carbondioxide detector may achieve a long shelf life (e.g. several years)without the use of a dessicant.

Additionally, the borosilicate substrate 12 may exhibit an improvedcolor cycling pattern in the presence of carbon dioxide. For example,with use of a common indicator solution 14, such as metacresol purple,the substrate 12 may change from a deep purple to a light tan color,rather than purple to yellow, in the presence of carbon dioxide. Oneadvantage of a purple-to-tan color change rather than a purple-to-yellowcolor change is that the contrast ratio between purple and tan isparticularly advantageous, allowing a healthcare worker to distinguishfiner gradations of carbon dioxide levels. Further, the purple-to-tancolor change is also helpful for people with color blindness, which mostoften impairs acuity in the green-yellow-red portion of the spectrum.

The performance of carbon dioxide detectors in humid air is significantto clinical use because exhaled breath contains considerable amounts ofwater. Thus, performance in humid conditions is indicative ofperformance with actual patients. It may affect the use-life of adetector. Accordingly, carbon dioxide detectors having a borosilicateand acrylic substrate show faster breath-to-breath response than thosehaving a cellulose fiber substrate such as paper. This faster responseis also facilitated by the highly porous nature of borosilicate, whichallows easier penetration of air than does a cellulose fiber substrate.This may indicate a longer use-life of the borosilicate substratedetector.

Color indicators may approximately match the color of the indicatorsolution in the presence of difference levels of carbon dioxide. Forexample, in one embodiment, a color indicator may reversibly changecolor from purple to yellow in the presence of sufficient levels ofcarbon dioxide. In one specific embodiment, a generally yellow color maycorrespond with normal expiration, while a generally purple color maycorrespond with normal inspiration. Color indicators may also includewritten or other visual information to allow a user to determine whatcarbon dioxide concentrations are indicated by various colors. Forexample, one portion of borosilicate sensing component 30 may show oneor various shades that correlate with a low carbon dioxideconcentration, such as below approximately 0.5% or between approximately0.03% and 0.5%. In such an embodiment, a borosilicate sensing component30 may contain shades of purple. Another portion of a borosilicatesensing component 30 may show one or various shades that correlate witha high carbon dioxide concentration typical of respired air, such asabove approximately 2% or between 2% and 5%. In such an embodiment, theborosilicate-sensing component 30 may contain shades of yellow. Anadditional portion of a borosilicate-sensing component 30 may indicatecarbon dioxide concentrations above that of normal or esophageal air,but below that corresponding with normal respiration. For example, aportion of a borosilicate-sensing component 30 may indicate carbondioxide concentrations common in respired air of a patient sufferingfrom perfusion failure. A portion of a borosilicate sensing component 30may show one or various shades that correlate with carbon dioxideconcentrations of between approximately 0.5% and 2%. In one specificembodiment, a portion of a borosilicate-sensing component 30 may containshades of grayish purple. Detection may include in-stream detection,such as in the current EasyCap™ (Nellcor, Tyco Healthcare, California)system. It may also include “side-stream” detection, such as in thecurrent INdCAP™ product (Nellcor, Tyco Healthcare, California). Thedetection system may be modified to facilitate either form of detection.

The borosilicate-sensing component 30 may be prepared by forming thesubstrate 44, then impregnating or coating it with the indicatorsolution. The substrate 44 may then be dried to immobilize the indicatorsolution on it. The substrate 44 may then be incorporated into an airwayproduct, as depicted in FIG. 2. During its formation and handling priorto packaging, the borosilicate-sensing component 30 may be kept inconditions to minimize or control chemical reactions that mightnegatively influence its reliability. For example, it may be kept in dryconditions after drying. Carbon dioxide detectors of the presentdisclosure may require less stringent pre-packaging conditions thancurrent cellulose filter paper detectors because of improvements inresistance to negative effects of humidity and room air. Carbon dioxidedetectors, detection systems, of further systems such as resuscitatorsmay be created in a sterile or clean environment or later sterilized.

The borosilicate-sensing component 30 may be used by exposing it torespiratory gases. The air then infiltrates the substrate 44 and anycarbon dioxide in the air reacts with the indicator solution. This mayproduce a color change in the indicator. Change of color back and forthbetween a low carbon dioxide color to a high color dioxide color mayindicate whether the patient is breathing normally. Change of color toone indicating low concentrations of carbon dioxide still aboveconcentrations in air may indicate perfusion failure in the patient.

In an alternative embodiment, a medical device may include a strip orother contiguous sensing component that provides information aboutcarbon dioxide at a plurality of locations along the device. As depictedin FIG. 3, an endotracheal tube 46 may include a borosilicate-sensingstrip 48 that is disposed along the inside passage of the conduit 32. Asexpired respiratory gases flow through the endotracheal tube 46, theareas of the strip where local concentration of carbon dioxide hasreached the critical threshold may change color. As the expired carbondioxide levels drop after the respiratory gas has moved through theairway, the color change may reverse. Thus, an airway device with acontiguous borosilicate-sensing strip may provide information aboutcarbon dioxide levels at a plurality of locations along a patientbreathing circuit.

In certain embodiments, a borosilicate-sensing component may beintegrated into the material of the airway device itself. For example,FIG. 4 illustrates a cross-sectional view of an exemplary conduit 50that may be incorporated into a patient breathing circuit. The conduit50 defines a passageway 60 through which gas may flow. The conduit 50includes a borosilicate-sensing layer 52 disposed between an inner layer56 and an outer layer 58. The borosilicate-sensing layer includesborosilicate fibers 54. The inner layer 56 and the outer layer 58 may beextruded over a borosilicate-sensing layer 52. The inner layer 56 may beformed from a material that is relatively permeable to carbon dioxide toallow the carbon dioxide in the passageway 60 to reach theborosilicate-sensing layer 52. The outer layer 58 may be formed from amaterial that is relatively impermeable to carbon dioxide in order toprevent egress of respiratory gases out of the airway device.

FIG. 5 illustrates an alternative embodiment of a conduit 62 thatincludes at least one embedded borosilicate-sensing portion 64 in amedical tubing structure 68. The borosilicate portion 64 includesborosilicate fibers 66. The conduit 62 defines a passageway 70. Depictedare two borosilicate-sensing portions 64 that may provide the advantageof a time-variable response. As the carbon dioxide in expiredrespiratory gas moves from the passageway through the sensing portions64, the indicator response time will be faster for areas of the sensingportions 64 adjacent to the passageway 70 and will be slower for areasof the sensing portions 64 closer to the outside of the conduit 62.Thus, the depth of the sensing portion 64 along the axis orthogonal tothe axis of the passageway 70 may slow down the response time of theindicator. In certain embodiments (not shown), the sensing portions 64may be sealed on the outside of the conduit 62 with a carbon dioxidebarrier to prevent egress of respiratory gases to the environment.

In an alternative embodiment, an endotracheal tube may be modified toallow for side sampling of respiratory gases at multiple locations alongthe tube. As depicted in FIG. 6, an endotracheal tube 72 may include afirst sampling lumen 74 located relatively closer to a distal end 36than a second sampling lumen. In such an embodiment, respiratory gasesare drawn into the sampling lumen and transferred to a distally locatedcarbon dioxide sensing component. In other embodiments (not shown) apatient breathing circuit may include side sampling lumens at anysuitable location. Such an embodiment may be advantageous as a distallylocated sensing component 10 may be easily replaced while the medicaldevice is still inserted into the patient.

In certain embodiments, carbon dioxide levels may be affected by thebuildup of oral-mucosal secretions, either in a conduit through whichrespiratory gases flow or on the outside of a device after it has beeninserted into a patient. As depicted in FIG. 7, an endotracheal tube 78may include carbon dioxide sensing elements 80 disposed adjacent to aproximal end of a cuff 34. The carbon dioxide sensing elements 80 mayprovide feedback to a monitor 11 when secretions build up on top of thecuff 34. In such an embodiment, the carbon dioxide sensing elements mayfacilitate detection of a baseline level of carbon dioxide directlyafter insertion of the endotracheal tube 78. The baseline level ofcarbon dioxide may represent a state in which minimal secretions arepresent. As secretions build up, the carbon dioxide sensing elements 80may be substantially covered by the secretions, and the measured carbondioxide levels will typically decrease in response to the presence ofthe secretions. In certain embodiments, a monitor 11 may providefeedback to a healthcare practitioner to indicate that secretions shouldbe aspirated from the cuff in response to the signal received from thecarbon dioxide sensing elements 80. Further, such aspiration may betriggered as an automated response to the decrease of detection ofambient carbon dioxide by the cuff.

In an alternative embodiment (not shown) a sensing component 10 may bedisposed on the inside of the endotracheal tube. In such an embodiment,a monitor 11 may include code operable to detect secretions byevaluating the spatial heterogeneity (variability) of carbon dioxideconcentration between multiple carbon dioxide sensing components 10disposed inside the endotracheal tube, at multiple spots along thelength of the tube and/or multiple angles inside its circumference.Further, the monitor 11 may provide an indication to the clinician tosuction secretions from the ET tube when it detects them, or the monitor11 may automatically initiate suctioning. Such a system may includefault-tolerant features that, automatically or with user input, mayoptionally disregard the signal from one or more of the plurality ofsensing components 10, for example if the secretions cannot be easily beremoved from that spot, so as to continue to provide reliable secretiondetection with a minimum of false alarms.

In certain situations, emergency healthcare workers may intubatepatients in the field. Often, tracheal intubation is impractical inthese situations because of certain time pressures as well as the levelof skill associated with tracheal intubation. As a result, emergencyhealthcare workers often use an esophageal tracheal airway device, suchas a Combitube® (available from Mallinckrodt, Pleasanton, Calif.), foremergency intubation. An esophageal tracheal airway device is adapted toenter either the esophagus or the trachea, allowing ventilation andoxygenation in both positions. Depicted in FIG. 8 is an improvedesophageal tracheal airway device 82 with carbon dioxide sensingelements 90 and 92 that provide immediate feedback as to which lumen ofthe dual-lumen esophageal tracheal airway device 82 is active. Asdepicted in FIG. 8, an esophageal tracheal airway device 82 generallyincludes a tracheal lumen 84 and an esophageal lumen 86 with a distalblocked end 89 and perforations 88 at the pharyngeal level. Theesophageal tracheal airway device 82 may also include an oropharyngealballoon 94 and a smaller balloon 96 in order to substantially seal thepatient airway. Once the esophageal tracheal airway device 82 has beeninserted into a patient, a healthcare worker may determine whether theesophageal tracheal airway device 82 entered the trachea or theesophagus by determining which lumen is active. For example, if carbondioxide sensing element 90 provides a visual indication of carbondioxide flow, a healthcare worker will know that the esophageal trachealairway device 82 was inserted into the trachea. Similarly, a positivecarbon dioxide visual signal from carbon dioxide sensing element 92indicates that the esophageal lumen 86 is active.

In an alternative embodiment (not shown), a carbon dioxide sensingelement 10 may be incorporated into each of the dual lumens of abreathing circuit designed for independent ventilation of one or both ofa patient's two lungs. In this case, having a carbon dioxide sensingelement 110 on each individual lumen that is specific to an individuallung would provide the advantage of enabling capnography for each lung,including estimation of dead space and changes therein for each lung.For example, independent lung ventilation may be advantageous if thepatient has a lung tumor, as surgeons may ventilate one lung whileexcising a tumor on the other. In other embodiments, damage to anindividual lung may result in independent ventilation of each lung atdifferent pressures. In certain embodiments each lumen may include aborosilicate sensing strip operatively connected to a monitor 11.

In certain embodiments, it may be advantageous to employ the techniquesprovided herein for capnography or volumetric capnography, which mayprovide information about carbon dioxide production, pulmonaryperfusion, alveolar ventilation, respiratory patterns and elimination ofcarbon dioxide from the anaesthesia circuit and ventilator. Typically,capnography involves measuring expiratory gas carbon dioxideconcentration against time during multiple respiratory cycles. Thecarbon dioxide levels measured by the present techniques may produce agraphical capnogram that may illustrate three phases in breath carbondioxide gas concentration during the patient exhale cycle in a healthypatient. The first phase indicates clearing of the conducting airwayswhich do not normally participate in gas exchange and which are referredto as “dead space.” The second phase typically involves exhalation ofair from conducting airways dynamically mixed with lung gases from theactive (alveoli) membrane surfaces within the lung that have undergonegas exchange with arterial blood. The third phase reflects theexhalation of unmixed gas from regions of the lung that are normally inactive exchange with the alveoli tissue.

Carbon dioxide concentration, when plotted against expired volume duringa respiratory cycle, is termed as volumetric canography. The volume ofcarbon dioxide exhaled per breath can be assessed. A volume capnogramprovides information about a variety of clinical states and may enhancethe information provided by a time capnogram. For example, a volumecapnogram may provide information about physiological dead space. Total“physiologic dead space” can therefore be measured using arterial carbondioxide and the Bohr equation. “Anatomic dead space,” which may includegas volume within a breathing circuit in which exhaled gas isrebreathed, such as the endotracheal tube, passive humidificationdevice, or Y-piece, can be calculated directly from the volumecapnogram. Alveolar dead space is the difference between physiologicdead space and anatomic dead space, and is related to the differencebetween alveolar and arterial carbon dioxide. An increase in physiologicdead space may indicate that the patient is at risk for pulmonaryembolism or pulmonary edema.

The present techniques may be employed in volumetric capnography. Forexample, delays between carbon dioxide changes detected by the sensingelements 10 in breathing circuit 12 could be used, during periods in thebreathing cycle when carbon dioxide is changing, to determine thevelocity of the gas. Such a calculation may also include inputting thedistance between the individual sensing elements 10. Gas flow throughthe breathing circuit 12 may be calculated by multiplying the velocityby the circuit cross-sectional area. Gas volume is calculated byintegrating flow over time. Anatomic dead space may be estimated as thetotal volume exhaled while carbon dioxide is increasing at the start ofexhalation. This method is advantageous in that it does not require aseparate flow sensor for this volumetric estimate.

The flow chart 100 depicted in FIG. 9 describes steps involved inmonitoring physiologic dead space, which involves acquisition of tissuecarbon dioxide data from the sensing elements 10 at step 104, and theacquisition of total breathing circuit volume data at step 102. Incertain embodiments, it is envisioned that steps 102 and 104 may occursimultaneously. In other embodiments, step 102 may be performed byselecting a predetermined setting on a monitor. For example, certaintypes of medical devices may be associated with a predetermined totalvolume. The total volume of the respiratory circuit may be determined bymultiplying the tube's, for example an endotracheal tube as well as itsassociated connecting tubing, cross-sectional area by its length. One ormore of these dimensions may be input by the user, or they may beestimated from other available information, such as patient weight.Alternatively, the volume of one or more circuit elements may bepredetermined at the time of manufacture and stored in a memory deviceembedded within the respiratory circuit 12 that may then be read by themonitor 11. At a step 106, a processor analyzes the carbon dioxide dataand total volume data to calculate physiologic dead space at eachsensing element 10 location.

If the volume capnogram at any individual location indicates an increasein physiologic dead space, control is passed to step 1 10, whichtriggers an alarm, which may be a visual or audio alarm, and to step112, which dictates that appropriate treatment protocols are instituted.For example, as physiologic dead space increases may be associated withpulmonary embolism, an appropriate treatment protocol may includeadministration of anticoagulants. In other embodiments, physiologic deadspace increases may be associated with respiratory events such as acuterespiratory distress syndrome, shock, sepsis, pneumonia, aspirationasthma attacks, lung injury, or lung collapse. Such events may correlatewith particular clinical patterns, which may include progressivehypoxemia, decreased lung compliance, intrapulmonary shunting, andnon-cardiogenic pulmonary edema. Particular clinical patterns may bedifferentiated from one another by patient history and also by uniquefeatures of the volumetric capnograms. If, at a step 108, the individualcarbon dioxide sensing elements 10 do not indicate any increase inphysiologic dead space, a processor passes control back to step 104.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Indeed, the presenttechniques may not only be applied to measurements of carbon dioxide,but these techniques may also be utilized for the measurement and/oranalysis of other respiratory gases. Rather, the invention is to coverall modifications, equivalents, and alternatives falling within thespirit and scope of the invention as defined by the following appendedclaims. It will be appreciated by those working in the art that sensorsfabricated using the presently disclosed and claimed techniques may beused in a wide variety of contexts. That is, while the invention hasprimarily been described in conjunction with the measurement of carbondioxide concentration in the airway, the airways products fabricatedusing the present method may be used to evaluate any number of sampletypes in a variety of industries, including fermentation technology,cell culture, and other biotechnology applications.

1. A multi-lumen intubation tube comprising: a conduit adapted totransfer gas to a patient's lungs comprising a first lumen and a secondlumen; a first carbon dioxide sensing component disposed on the firstlumen; and a second carbon dioxide sensing component disposed on thesecond lumen.
 2. The medical device of claim 1, comprising a ventilatorto which the conduit is operatively connected.
 3. The medical device ofclaim 1, comprising a selective barrier disposed on at least one sensingcomponent that is substantially impermeable to water.
 4. The medicaldevice of claim 1, comprising a temperature sensor adapted to providesignal related to a gas temperature, a humidity sensor, or an ambientpressure sensor.
 5. The medical device of claim 1, wherein at least oneof the sensing components comprises a chemical indicator.
 6. The medicaldevice of claim 1, wherein at least one of the sensing componentscomprises an electrochemical transducer.
 7. The medical device of claim1, wherein at least one of the sensing components comprises anon-optical transducer.
 8. The medical device of claim 1, wherein atleast one of the plurality of sensing components comprises an opticaltransducer.
 9. The medical device of claim 1, wherein at least one ofthe sensing components comprises a quantum-restricted element.
 10. Themedical device of claim 1, wherein at least one of the sensingcomponents comprises a borosilicate substrate and a carbon dioxideresponsive indicator solution disposed on the borosilicate substrate.11. The medical device of claim 10, wherein the borosilicate substratecomprises an acrylic binder.
 12. The medical device of claim 1, whereinat least one of the sensing components is embedded in the conduit.
 13. Amethod of manufacturing a multi-lumen intubation tube comprising:providing a conduit adapted to transfer gas to a patient's lungscomprising a first lumen and a second lumen; providing a carbon dioxidesensing element disposed on the first lumen; and providing a carbondioxide sensing element disposed on the second lumen.
 14. The method ofclaim 13, comprising providing a selective barrier disposed on at leastone sensing component that is substantially impermeable to water. 15.The method of claim 13, comprising providing a temperature sensoradapted to provide signal related to a gas temperature, a humiditysensor, or an ambient pressure sensor.
 16. The method of claim 13,wherein providing the first sensing component or the second sensingcomponent comprises providing at least one sensing component comprisinga chemical indicator.
 17. The method of claim 13, wherein providing thefirst sensing component or the second sensing component comprisesproviding at least one sensing component comprising an electrochemicaltransducer.
 18. The method of claim 13, wherein providing the firstsensing component or the second sensing component comprises providing atleast one sensing component comprising a non-optical transducer.
 19. Themethod of claim 13, wherein providing the first sensing component or thesecond sensing component comprises providing at least one sensingcomponent comprising an optical transducer.
 20. The method of claim 13,wherein providing the first sensing component or the second sensingcomponent comprises providing at least one sensing component comprisinga quantum-restricted element.
 21. The method of claim 13, whereinproviding the first sensing component or the second sensing componentcomprises: providing at least one sensing component comprising aborosilicate substrate; and providing a carbon dioxide responsiveindicator solution disposed on the borosilicate substrate.
 22. Themethod of claim 13, wherein providing the first sensing component or thesecond sensing component comprises embedding at least one of theplurality of sensing components is in the conduit.
 23. A method ofdetermining which lumen is active in a multi-lumen tube comprising:inserting a multi-lumen tube into a patient's airway; determining aconcentration of carbon dioxide gas at a location in a first lumen; anddetermining a concentration of carbon dioxide gas at a location in asecond lumen, wherein the lumen with the higher concentration of carbondioxide gas is the active lumen.
 24. The method of claim 23, comprisingtransferring gas to the patient through the active lumen.